I find it difficult to actually collaborate with other writers. Sharing ideas and coming up with premises is fine, but working together on a single narrative rarely ends well. I think it is a combination a desire for control (usually from both parties), and a difference in visualization of where the story goes.
I have an idea for a story about a man who is very interested in solving puzzles. As a challenge, he teaches himself how to pick locks. From there, he increases the challenge by picking locks of people's homes at night without getting caught. But where do I go from there?
My thought is to have him break into people's houses and walk around without being caught, just as another challenge, but he becomes so racked with guilt that he leaves little presents as an anonymous apology, leading the town to think of this mysterious person as a Santa Claus/Reverse Cat Burglar.
Another's thought is to have him continue to increase the challenge, becoming more devious and more daring with each challenge, until he finally meets his match.
A third person's thought is to have him use his powers for good and become some sort of secret agent like James Bond and becoming a classic type hero.
These are not reconcilable ideas. They can't blend together. One of them has to be chosen. And from there, the story becomes that person's story. The other co-writers are more like helpers. And that's where the power issue comes in. If everybody is working on your idea, you don't want them mutilating your baby.
It's difficult to play in other people's sandbox. It's difficult to share. I'm sure that when working with reasonable people, it is not too hard to learn to be reasonable, or how to work on projects fairly and evenly. But it will never be a walk in the park. Imagine the difficulty of writing by yourself. Now imagine three people trying to write by themselves. Not gonna be pretty.